Unable To Communicate With The EBCM DTC U0121 - keywords ABS brake IPC lamp light program stabilitrak TCS

This PI was superseded to update model years. Please discard PIT4056B.

The following diagnosis might be helpful if the vehicle exhibits the symptom(s) described in this PI.

Some owners may comment that the ABS and Red Brake MIL is on in the IPC. When trying to retrieve DTCs the Tech 2 will not communicate with the EBCM. The Tech 2 will communicate with all other High and Low speed GM LAN modules and some may have set a U0121 DTC against the EBCM. While doing normal diagnostics it will be found that the power circuits 1640 and 442, ground circuit 2150 (2), enable signal circuit 5986 and GM LAN circuit 2500 and 2501 to the EBCM test good. This testing may lead to replacement of the EBCM, however, the concern will still be present after installing the new EBCM.

This concern may be caused by the Yaw Rate and Lateral Acceleration Sensor Supply Voltage, circuit 2087 Dk Green shorted, pulling down the internal circuitry of the EBCM. If circuit 2087 is backed out of the EBCM connector and communication returns, inspect for this circuit shorted to ground or yaw sensor concern. On some utility vehicles the circuit may be shorted under the passenger front seat at the rear camera module bracket. Repair the circuit as necessary and reroute to prevent the concern from reoccurring.

Please follow this diagnostic or repair process thoroughly and complete each step. If the condition exhibited is resolved without completing every step, the remaining steps do not need to be performed.